4-Stroke Spark-Ignition Engines
Premixed Internal Combustion Engines with Spark Ignition intake
exhaust
2-StrokeEngines
intake
Reed Valve 2-stroke
Wankel Engines exhaust +: No valves needed Continuous motion Æ less vibration -: Leaks through seals Æ low compression ratio Æ pollution (high levels of HC and CO)
intake
4-Stroke SI Engines
Emission characteristics of air/fuel ratio
Otto cycle
Topics of Engine Combustion • Brief introduction: - Spark Ignition (gasoline) - Fuel preparation, Ignition, Power analysis (efficiency & losses) , Engine Knock, Modeling, Emissions • Areas of improvement • Emerging new technologies [Lean Burn, Homogeneous Charge Compression Ignition (HCCI) ]
SI Engine Performance VS Engine RPM Torque Torque
Power Power
SFC
Power (kw) =
2π * Torque( NM ) * RPM 60,000
Specific Fuel Consumption •
Engine RPM
SFC =
m fuel Power Output
Spark Ignition (SI) Engines: • Control of combustion: ignition timing
• - Low efficiency (low compression ratio, limited by knocking) • + Cleanest emission with help of 3-way catalyst • Advanced Technologies: --New fuel with higher octane number, improving combustion efficiency, reduces losses (pumping, heat, friction, exhaust & intake)
SI Combustion
-Spark initiates combustion -Turbulent flame propagation -Stoichiometric mixture -High combustion temperature -Compression ratio Limited by autoignition
Understanding Engine Knock Some physical models in mind: • Unburn gas is compressed by bunred turbulent flame leading high temperature and pressure • Unburn gas auto-ignites or ignition triggered by hot spot. • Autoignition delay time is of Pressure waves bounce critical importance back and forward throughout cylinder
Autoignition delays of large-molecule fuels Exhibit negative temperature behaviors n-heptane: C7H16 Main ignition Low temp. ignition
T ↑ delay ↓
Negative temperature behaviors: T ↑ delay ↓
ÆNegative temperature behaviors complicate engine designs
Autoignition delays of large-molecule fuels Exhibit negative temperature behaviors ition Delay [ms]
10000 Isooctane Methane PRF ethanol
Methane
Ethano Isoo
100 PRF
1
Fuel Octane number(RON) CH4 Methane 120 C2H5OH Ethanol 107 C8H18 Isooctane 100 PRF(80) 80
-Methane and ethanol do not show “negative” temperature in the IC engine application regime •Are oxygenated fuels (biofuels) similar to ethanol? • Predictive models?
Guidance to SI Engine Design To avoid autoignition so that a higher compression ratio can be usedÆ higher thermal efficiency • reduces time available for unburned gas -- higher flame speed (controlled largely by turbulence) -- multiple ignition sources (limited by space) • Keep unbuned gas as cool as possible • Fuel additives (small amount of ethanol and others) … All these are explored by engine tests with test matrix assisted by CFD simulations
Multiple Site Spark Ignition Benefits: • Combustion is initiated at outer and propagating inward • Fast combustion rate eliminating end gas auto-ignition • Higher combustion efficiency • Shorter combustion period less heat loss
3D-CFD Predictions of Engine Knock
Combustion Chamber Geometry • Enhances turbulent combustion speed to decrease burning time • Minimizes heat transfer & pollution formation • Needs to be integrated with valve timing and intake manifold designs • CFD calculations
Candidate Geometries
CFD of Intake Flows Velocity Field
Turbulence Intensity
Turbulent Premixed Flames
Flame Development Bunsen Turbulent Flame
Internal Combustion Engine
Turbulent Flames
Empirical Relation:
0.82
⎛ p ⎞ ⎜⎜ ⎟⎟ 1 + 0.05θ 0.4 ⎝ pm ⎠ u ' = turbulent velocity fluctuations ST u' = 1 + 1.21 SL SL
(
pm = motoring pressure
θ = spark ignition before TDC
)
Enhancing turbulence by multiple valve arrangement Vertical Vortex – generates strong Turbulence as the tumble is broken up near the top dead center ÆFast burn ÆLess time for heat transfer ÆPossible higher compression ratio Æ10-20% better fuel economy
Various Losses from SI Engines Ideal Otto Cycle Time loss (slow combustion)
Heat loss
Exhaust Blowdown loss
Pumping loss
Sketch of a Carburetor
Fuel Injection System
Throttle
ECU: Electronic Control Unit
Pressure data from a 1.8 Liter Pontiac Engine at Berkeley
Pumping losses become significant
Overall Performance from SI Engines with/without Throttling
Efficiency
Self-ignition unthrottled Current throttled SI engines
Effective Mean Pressure[bar]
Importance of valve timing
Gasoline Direct Injection (GDI)Lean Burn Engines Advantages: -lean burn -Low (or no) pumping loss -High compression ratio -High efficiency Disadvantages: -hard to maintain good combustion -high NOx an HC emissions -special NOx absorption catalyst
Enabling Technologies: • Combustion Chamber Geometry • Combustion of Diluted Mixtures • Multiple source ignition • Exhaust energy retention • Friction Reduction
• Optimal control of Spark timing or Injection • Variable Valve Timing (VVT) •Variable Compression Areas offering potentially large improvement in engine performance for current & future fuels
Æ HCCI Technologies
Modeling of MON, RON and HCCI Number Complete Model with CFD good for engine design but too expensive with detailed chemical kinetics removed
ÆChemical kinetics base model -- Single zone well-mixed reactor Æ HCCI -- Two zone model (or multizone shell) Æ SI MON and RON
Complete CFD with detailed chemistry exceeds Current computer capacities
Emission Characteristics from IC Engines
Emission as function of air/fuel ratio
a) gasoline engine b) diesel engine
NOx Emission is highly sensitive to temperature
Sources of Unburned Hydrocarbon and CO
Unburned HC and CO are stored in crevices and released during expansion stroke as wall jets.
Spark Ignition (SI) Pontiac Engine
Engine performances: -- Pressure transducer -- torque -- SFC -- emission before and after catalyst
Horiba gas analyzers: CO,HC, NOx, O2, CO2
Comparison: 3 Methods of Internal Combustion Engines
Gasoline (Premixed Spark)
Diesel (Direct Injection)
HCCI (Premixed Autoignition of lean mixtures)
What is HCCI ? • Homogeneous-Charged Compression Ignition (HCCI) engine -- has advantages of both SI and CI engines •
Global autoignition of premixed fuel and air (High compression ratio) • Heat release controlled by chemical kinetics • No flame propagation - operation possible at low Φ*
• Benefits and Challenges •
•
Benefits: 1. Low combustion temperature (lean mixture < Φ~0.5) Æ lower NOx emissions 2. Premixed charge Æ no soot 3. High compression ratio Æ high thermal efficiency Challenges: 1. Control of Start of Combustion (SOC) is the main issue in HCCI (No direct control, i.e. spark or fuel injector) 2. High unburned hydrocarbon and CO emissions * equivalence ratio for fuel-air mixture
exhaust
2-StrokeEngines
intake
Reed Valve 2-stroke
Wankel Engines exhaust +: No valves needed Continuous motion Æ less vibration -: Leaks through seals Æ low compression ratio Æ pollution (high levels of HC and CO)
intake
4-Stroke SI Engines
Emission characteristics of air/fuel ratio
Otto cycle
Topics of Engine Combustion • Brief introduction: - Spark Ignition (gasoline) - Fuel preparation, Ignition, Power analysis (efficiency & losses) , Engine Knock, Modeling, Emissions • Areas of improvement • Emerging new technologies [Lean Burn, Homogeneous Charge Compression Ignition (HCCI) ]
SI Engine Performance VS Engine RPM Torque Torque
Power Power
SFC
Power (kw) =
2π * Torque( NM ) * RPM 60,000
Specific Fuel Consumption •
Engine RPM
SFC =
m fuel Power Output
Spark Ignition (SI) Engines: • Control of combustion: ignition timing
• - Low efficiency (low compression ratio, limited by knocking) • + Cleanest emission with help of 3-way catalyst • Advanced Technologies: --New fuel with higher octane number, improving combustion efficiency, reduces losses (pumping, heat, friction, exhaust & intake)
SI Combustion
-Spark initiates combustion -Turbulent flame propagation -Stoichiometric mixture -High combustion temperature -Compression ratio Limited by autoignition
Understanding Engine Knock Some physical models in mind: • Unburn gas is compressed by bunred turbulent flame leading high temperature and pressure • Unburn gas auto-ignites or ignition triggered by hot spot. • Autoignition delay time is of Pressure waves bounce critical importance back and forward throughout cylinder
Autoignition delays of large-molecule fuels Exhibit negative temperature behaviors n-heptane: C7H16 Main ignition Low temp. ignition
T ↑ delay ↓
Negative temperature behaviors: T ↑ delay ↓
ÆNegative temperature behaviors complicate engine designs
Autoignition delays of large-molecule fuels Exhibit negative temperature behaviors ition Delay [ms]
10000 Isooctane Methane PRF ethanol
Methane
Ethano Isoo
100 PRF
1
Fuel Octane number(RON) CH4 Methane 120 C2H5OH Ethanol 107 C8H18 Isooctane 100 PRF(80) 80
-Methane and ethanol do not show “negative” temperature in the IC engine application regime •Are oxygenated fuels (biofuels) similar to ethanol? • Predictive models?
Guidance to SI Engine Design To avoid autoignition so that a higher compression ratio can be usedÆ higher thermal efficiency • reduces time available for unburned gas -- higher flame speed (controlled largely by turbulence) -- multiple ignition sources (limited by space) • Keep unbuned gas as cool as possible • Fuel additives (small amount of ethanol and others) … All these are explored by engine tests with test matrix assisted by CFD simulations
Multiple Site Spark Ignition Benefits: • Combustion is initiated at outer and propagating inward • Fast combustion rate eliminating end gas auto-ignition • Higher combustion efficiency • Shorter combustion period less heat loss
3D-CFD Predictions of Engine Knock
Combustion Chamber Geometry • Enhances turbulent combustion speed to decrease burning time • Minimizes heat transfer & pollution formation • Needs to be integrated with valve timing and intake manifold designs • CFD calculations
Candidate Geometries
CFD of Intake Flows Velocity Field
Turbulence Intensity
Turbulent Premixed Flames
Flame Development Bunsen Turbulent Flame
Internal Combustion Engine
Turbulent Flames
Empirical Relation:
0.82
⎛ p ⎞ ⎜⎜ ⎟⎟ 1 + 0.05θ 0.4 ⎝ pm ⎠ u ' = turbulent velocity fluctuations ST u' = 1 + 1.21 SL SL
(
pm = motoring pressure
θ = spark ignition before TDC
)
Enhancing turbulence by multiple valve arrangement Vertical Vortex – generates strong Turbulence as the tumble is broken up near the top dead center ÆFast burn ÆLess time for heat transfer ÆPossible higher compression ratio Æ10-20% better fuel economy
Various Losses from SI Engines Ideal Otto Cycle Time loss (slow combustion)
Heat loss
Exhaust Blowdown loss
Pumping loss
Sketch of a Carburetor
Fuel Injection System
Throttle
ECU: Electronic Control Unit
Pressure data from a 1.8 Liter Pontiac Engine at Berkeley
Pumping losses become significant
Overall Performance from SI Engines with/without Throttling
Efficiency
Self-ignition unthrottled Current throttled SI engines
Effective Mean Pressure[bar]
Importance of valve timing
Gasoline Direct Injection (GDI)Lean Burn Engines Advantages: -lean burn -Low (or no) pumping loss -High compression ratio -High efficiency Disadvantages: -hard to maintain good combustion -high NOx an HC emissions -special NOx absorption catalyst
Enabling Technologies: • Combustion Chamber Geometry • Combustion of Diluted Mixtures • Multiple source ignition • Exhaust energy retention • Friction Reduction
• Optimal control of Spark timing or Injection • Variable Valve Timing (VVT) •Variable Compression Areas offering potentially large improvement in engine performance for current & future fuels
Æ HCCI Technologies
Modeling of MON, RON and HCCI Number Complete Model with CFD good for engine design but too expensive with detailed chemical kinetics removed
ÆChemical kinetics base model -- Single zone well-mixed reactor Æ HCCI -- Two zone model (or multizone shell) Æ SI MON and RON
Complete CFD with detailed chemistry exceeds Current computer capacities
Emission Characteristics from IC Engines
Emission as function of air/fuel ratio
a) gasoline engine b) diesel engine
NOx Emission is highly sensitive to temperature
Sources of Unburned Hydrocarbon and CO
Unburned HC and CO are stored in crevices and released during expansion stroke as wall jets.
Spark Ignition (SI) Pontiac Engine
Engine performances: -- Pressure transducer -- torque -- SFC -- emission before and after catalyst
Horiba gas analyzers: CO,HC, NOx, O2, CO2
Comparison: 3 Methods of Internal Combustion Engines
Gasoline (Premixed Spark)
Diesel (Direct Injection)
HCCI (Premixed Autoignition of lean mixtures)
What is HCCI ? • Homogeneous-Charged Compression Ignition (HCCI) engine -- has advantages of both SI and CI engines •
Global autoignition of premixed fuel and air (High compression ratio) • Heat release controlled by chemical kinetics • No flame propagation - operation possible at low Φ*
• Benefits and Challenges •
•
Benefits: 1. Low combustion temperature (lean mixture < Φ~0.5) Æ lower NOx emissions 2. Premixed charge Æ no soot 3. High compression ratio Æ high thermal efficiency Challenges: 1. Control of Start of Combustion (SOC) is the main issue in HCCI (No direct control, i.e. spark or fuel injector) 2. High unburned hydrocarbon and CO emissions * equivalence ratio for fuel-air mixture
